Even though there are significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties of practical usefulness of the micromirrors due to the reliability problems caused by the limited life time of the micromirror hinges employed to support the micromirrors. Specifically, the metal hinges commonly implemented in the commercially available micromirrors tend to have mechanical fatigue and creep phenomena especially when operated in an elevated temperature. Manufacturers chose metals to make hinges for supporting the micromirrors because the simpler and more convenient processes for manufacturing the metal hinges.
Metal wires cannot be eliminated because they are necessary to control MOS drivers under mirrors and hinges. Once metal lines are formed, high temperature processes applied to the wafers are not permissible due to the concerns that the metal lines will melt or damaged at elevated temperatures. For example, aluminum is typically used for wiring. A temperature higher than 400 Celsius applied to the wafer is likely to cause damages to the aluminum wires. Aluminum will be damaged over 400 degree C. It is very natural for the DMD manufacturer to choose metal for hinges and mirrors, because it does not require higher temperature than 400 degree C. to form hinges and mirrors. Conversely, high temperature processes are often required to strengthen the hinges made of a semiconductor. For these reasons, the difficulties of unreliable metal hinges that often encountered in the conventional micromirror device cannot be easily resolved.
There is a great demand to overcome such problems. Specifically, the field of micro-electromechanical systems (MEMS) has experienced an explosive growth during the last decade having found applications in accelerometers, pressure sensors, actuators and the miniaturization of various other mechanical devices. Electromechanical micromirror devices are an example of a MEMS device that has drawn considerable interest because of their application as spatial light modulators (SLMs).
A spatial light modulator requires an array of a relatively large number of such micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. A digital micro-mirror device (DMD) is one example of a micro-mechanical SLM. One or more hinges support each mirror and allow the mirrors to tilt. Images are formed by selectively tilting each mirror to reflect or not reflect an incident source of light onto an image plane.
In a typical video application, each mirror is expected to switch over thousand times per second. Therefore, the design and material of the hinge is critical to DMD reliability.
The main reliability concern regarding the hinge is plastic deformation. Through continued use and operation in extreme temperatures, the hinge undergoes mechanical deformation, also known as creep. The relaxation of the hinge results in a residual tilt when all voltages are removed. This, so called hinge memory, is discussed in Douglass, “Lifetime Estimates and Unique Failure Mechanisms of the Digital Micromirror Device,” IEEE International Reliability Physics Symposium, 36th Annual, pp. 9-16, April 1998. As discussed in this paper, mirrors will not function properly when the residual tilt exceeds approximately 35 to 40% of the 10-degree rotation angle. In addition, while duty-cycle contributes to creep, the dominant factor for hinge memory lifetime is temperature.
In U.S. Pat. No. 5,142,405, the mirror is tilted by means of an electrostatic force created by biasing the mirror and address electrodes appropriately. The advantage of biasing the mirror is that a lower address voltage can be used to achieve electrostatic motion. Through the use of the appropriate mirror bias, bistable operation can be achieved with standard 5V CMOS address circuitry. The address voltage applied, however, requires enough operating margin to compensate for the residual tilt resulting in further design complexity.
U.S. Pat. No. 5,083,857 describes a DMD pixel architecture that improves contrast and brightness by placing the hinge and mirror support post under the rotatable mirror surface. The hinge, however, is composed of an aluminum alloy that is highly susceptible to metal creep. In addition, the support post connecting the hinge to the mirror forms a recess on the surface of the mirror. This recess is defined by the edges of the support post and is also known as a spacervia. The edges of the spacervia diffract the incident light into the projection system optics when the mirrors are tilted to the off state, thus limiting the pixel architecture's improvement to contrast ratio. This diffraction effect is known as Fraunhofer diffraction.
U.S. Pat. No. 6,038,056 improves upon the prior art of U.S. Pat. No. 5,083,857 by reducing Fraunhofer diffraction resulting from the support post edges. This is accomplished by reducing the dimensions of the support post edges and orienting the support post edges and mirror edges to be parallel to each other and at 45-degrees with respect to the incident light.
U.S. Pat. No. 5,631,782 and U.S. Pat. No. 6,447,126 describe a mirror support pillar in which the top of the pillar is covered and closed. This improvement eliminates the recess on the mirror surface of prior art and thus provides a method to eliminate the diffraction due to spacervias. However, this process cannot produce an optically flat mirror since the underlying spacer layer is not flat.
U.S. Pat. No. 5,652,671 also improves upon the prior art by proposing a hinge fabricated from alternating layers of different materials. While this reduces the hinge memory by providing a more elastic hinge, it does not eliminate it. Furthermore, the formation of the multi-layer hinge results in a more complicated manufacturing process as compared to a hinge made of a single material.
Alternatives to hinges composed of metal alloys are hinges composed of semiconductor material. Silicon is the dominant material used throughout the IC industry today. Furthermore, single crystal silicon is considered a perfect elastic material at normal temperatures. As discussed in Gad-el-Hak, M., ed., The MEMS Handbook, Boca Raton, CRC Press, 2002, pg. 16-23, silicon exhibits no plastic deformation or creep below 800 degrees Celsius. In addition, impurity atoms, also known as dopants, can be introduced into the semiconductor thereby altering its electrical properties. The result is a doped semiconductor in which its conductivity can be controlled by dopant concentration. These characteristics offer significant advantages over aluminum alloy hinges in both reliability and manufacturing complexity.
US 20030234994 describes a reflective SLM in which the hinge is composed of doped silicon and the mirror is biased appropriately to achieve electrostatic deflection under a 5V CMOS design. US 20040141894 also describes a micromirror unit composed of doped silicon. These and other prior art utilizing doped semiconductors for their hinge material fail to provide a device architecture in which the hinge is hidden from incoming light. This is an important disadvantage which results in poor contrast and fill-factor in applications such as image projection.
Despite significant advances that have been made in recent years, there is still a need for improvement in the performance and reliability of these hinges. Specifically, there is a need in the art for a conductive hinge that is less complex to manufacture and not susceptible to creep under typical to extreme temperatures. In addition, there exists a need for a hinge architecture that facilitates the fabrication of an optically flat mirror and hidden hinge thus eliminating Fraunhofer diffraction while improving contrast and fill-factor.
Furthermore, there is a need to improve the manufacturing processes to fabricate hinges with strengthened operational characteristics by applying processes that apply elevated temperatures. The improved high temperature processes must be employed without causing damages to the metal wires required for transmitting signals from the control circuits to control the operations of the micromirrors.